Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Dramatic Reduction of Gate Leakage Current of Ultrathin Oxides through Oxide Structure Modification
ZhiZhi ChenChen, Jun , Jun GuoGuo, and , and ChandanChandan B. B. SamantaraySamantaray
Department of Electrical and Computer Engineering Department of Electrical and Computer Engineering University of KentuckyUniversity of KentuckyLexington, KY 40506Lexington, KY 40506
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
OutlineOutline
•• Theory of Hydrogen/Deuterium Isotope Effect Theory of Hydrogen/Deuterium Isotope Effect
•• Experimental Evidence for Origin of Isotope EffectExperimental Evidence for Origin of Isotope Effect
•• Discovery of Phonon EnergyDiscovery of Phonon Energy--Coupling EnhancementCoupling Enhancement
•• Dramatic Improvement of Quality of Gate OxidesDramatic Improvement of Quality of Gate Oxides
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
e-e-
Si
H
H
SiSi
Si Si Si
Si
H
H
H
Si
SiSiSi Si
SiSi
SiO2
Sie- e-
Si
SiSi
Si
Si Si
Hot electr ons desorb hydr ogen, cr eating interface states which degrade device performance.
Hydrogen Passivated MOSFETHydrogen Passivated MOSFET
O
O
O O
O
O
O
O
O O
O
O
O Si Si
e-
SiO2SiO2 n+n+
p-Si substratep-Si substrate
Polysilicon Gate Deposited
Insulator
Metal Drain Contact
Metal Source Contact
Hot Electrons near Drain
n-n-
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Van de Walle & Jackson Theory(Van de Walle et al., Appl. Phys. Lett. vol. 69, 2441 (1996))
Two competing processes:Two competing processes:
••Hot electron excitation causes Hot electron excitation causes SiSi--H/D bond breaking.H/D bond breaking.
••DeDe--excitation is due to energy coupling from excitation is due to energy coupling from SiSi--D to phonon. D to phonon.
Reason:Reason: νν ∝∝ 1/m based on1/m based on IR spectroscopy theoryIR spectroscopy theory
SiSi--H H vibrationalvibrational frequency frequency νν ~ 650 ~ 650 cm-1.SiSi--D D vibrationalvibrational frequency frequency ≈≈ 460 cm460 cm--11 ((Si-Si TO phonon mode)
DeDe--excitation is more efficient for excitation is more efficient for SiSi--D bonds than for D bonds than for SiSi--H bondsH bonds------This is why This is why SiSi--D bonds are stronger than D bonds are stronger than SiSi--H ones. H ones.
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Schematic of Hydrogen/Deuterium Effect: Energy coupling from Si-D bending mode to Si-Si TO phonon mode
No coupling from Si-H bending mode to Si-Si TO phonon mode
OO
SiSi
SiSiSiSi
SiSi
SiSi
SiSi
SiSi
SiSiSiSi
SiSiOO
OO
OO
DD HHSiOSiO22
Energy CouplingEnergy Coupling
SiSi
No CouplingNo Coupling
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Direct Measurement of the Vibrational Frequency of Si-H/D Bonds
No experimental data available for No experimental data available for SiSi--D D vibrationalvibrational frequency in the frequency in the SiOSiO22/Si interface/Si interface
Only in the deuterated amorphous Si (α-Si), the vibrationalfrequency (510 cm-1) was measured*. However, the chemical environment of the amorphous Si is very different from that of crystal Si.
*J.-H. Wei, M.-S. Sun, and S.-C. Lee, Appl. Phys. Lett. 71, 1498 (1997).
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Origin of the Isotope Effect: No Energy Coupling from Si-H to Si-Si TO phonon
0.000
0.002
0.004
0.006
0.008
0.010
425475525575625675725775825875
Wavenumber (cm-1)
Abs
orba
nce
Si/oxide, hydrogen annealed
Si/oxide, unannealed
Si-OLO
Si-Si TOX4 point
Si-O TOrocking
Si-Si LO
Si-HBending
Si-O TObending
No difference between the H-annealed sample and the as-oxidized one, except for the Si-H bending vibration. (Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Origin of the Isotope Effect: Energy Coupling from Si-D to Si-SiTO phonon & Si-O TO rocking mode (Chen et al. Appl. Phys. Lett. )
0 .0 0 0
0 .0 0 2
0 .0 0 4
0 .0 0 6
0 .0 0 8
0 .0 1 0
0 .0 1 2
4 1 54 6 55 1 55 6 56 1 56 6 57 1 57 6 58 1 58 6 5
W a v e n u m b e r (c m -1)
Abs
orba
nce
S i/o x id e , u n a n n e a le d
S i/o x id e , d e u te riu m
S i-O T Ob e n d in g
S i-O T Oro c k in g
S i-S i L O
S i-S i T OX 4 p o in t
S i-O L O
S i-D B e n d in g
The absorbance of the Si-Si TO phonon mode and the Si-O TO rocking mode are all enhanced significantly (>25%) after deuterium anneal. (Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
New finding: Energy is coupled from Si-D bending mode to Si-Si TO phonon mode and also to Si-O TO rocking mode
(Chen et al. Appl. Phys. Lett. 83, 2151-2153, (2003))
OO
SiSi
SiSiSiSi
SiSi
SiSi
SiSi
SiSi
SiSiSiSi
SiSiOO
OO
OO
DD HHSiOSiO22
Energy CouplingEnergy Coupling
SiSi
No CouplingNo Coupling
Energy CouplingEnergy Coupling
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Challenge: How to further enhance the energy coupling?
Hypothesis:Hypothesis: Shift the Shift the SiSi--D D vibrationalvibrational mode toward mode toward SiSi--SiSi TO phonon mode.TO phonon mode.
Method 1: Mechanical stressjust a little shift (~6-8 cm-1)
Method 2: Electrical stress
Method 3: Thermal stress How?
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Surprising Discovery: Phonon Energy-Coupling Enhancement
The absorbance of the Si-Si TO phonon mode,the Si-O TO rocking mode, and Si-Si LO mode are all enhanced significantly (>50%) after rapid thermal processing (RTP). There is further enhancement after deuterium annealing. Tox=23 nm.
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
415440465490515540565590615640Wavenumbers(cm-1)
Abs
orba
nce
1. Si/Oxide, Unannealed
2. Si/Oxide, RTP annealed
3. Si/Oxide, RTP plus D2annealed
Si-O TOrocking
Si-Si LO
Si-Si TOX4 point
Si-D Bending
1
2
3
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
The Enhancement is not due to the Surface Plasmon.It is well-known that the surface plasmon on the nanoscale metallic islands also produces strong surface-enhanced IR spectra. In order to avoid the metallic island-like surface, we used n- wafer (n=2×1014 cm-3 and ρ=20.8 Ω-cm) for experiments.
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
415435455475495515535555575595615635Wavenumbers(cm-1)
Abs
orba
nce 1. Si/Oxide, Unannealed
2. Si/Oxide, RTP annealed
1
2
Si-Si TOX4 point
Si-O TOrocking
Si-Si LO
Si-O LO
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Dependence of Enhancement on the Oxide ThicknessFor thick oxide (Tox=80 nm), there is almost no enhancement except for the Si-Si LO mode after rapid thermal processing (RTP)---- implying stress-related phenomena. This also suggests that there should be no effect for the polysilicon/oxide stack.
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
410430450470490510530550570590610630650
Wavenumbers(cm-1)
Abs
orba
nce
Si/Oxide, UnannealedSi/Oxide, RTP annealed Si-O TO
rocking
Si-Si LO
Si-Si TOX4 point
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Dependence of Enhancement on the Cooling Time
The enhancement is strongly dependent on the cooling time ----implying the stress-related phenomena.
0.0000.0020.0040.0060.0080.0100.0120.0140.0160.018
430450470490510530550570590610630650Wavenumbers(cm-1)
Abs
orba
nce
1. Si/Oxide, Unannealed
2. Si/Oxide, RTP annealed,cooling 10 minutes
3. Si/Oxide, RTP annealed,cooling 5 minutes
4. Si/Oxide, RTP annealed,cooling 20 seconds
1
2
3
4
Si-Si TOX4 point
Si-O TOrocking
Si-Si LOSi-O
LO
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Phonon Energy-Coupling Enhancement: improvement of hot-electron degradation
-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
100 101 102 103 104 105
L=2um & W=150 um, Stressed at Vg=4V and Vd=8V
tStress Time (s)
D2 anneal only
RTP and D2 anneal
200X
-10
0
10
20
30
40
50
100 101 102 103 104
D2 anneal only, stress Vg=4V Vd=8VRTP+D2 anneal, stress at Vg=9V Vd=18V
Time (s)
Z. Chen and J. Z. Chen and J. GuoGuo, presented at the 35th IEEE SISC, San Diego, CA, Dec. 9, presented at the 35th IEEE SISC, San Diego, CA, Dec. 9--11, 2004.11, 2004.
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Hypothesis: Si-O bonds might be strengthened.This is because energy is also coupled from Si-O rocking mode to Si-Si TO
phonon mode and also to Si-D bending mode
OO
SiSi
SiSiSiSi
SiSi
SiSi
SiSi
SiSi
SiSiSiSi
SiSiOO
OO
OO
DDSiOSiO22
Energy CouplingEnergy Coupling
SiSi
Energy CouplingEnergy Coupling
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Hydrogen/Deuterium Effect on Gate Oxide: No Effect
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 2 4 6 8 10 12
D2 anneal
D2 anneal
D2 anneal
H2 anneal
H2 anneal
Gate Voltage (V)
10-3
10-4
10-5
10-6
10-7
10-8
10-2
Tox
=10.2nm
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Direct Rapid Thermal Processing Only: Improvement of Breakdown Voltage (15%) and Reduction of Leakage Current (10X)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 2 4 6 8 10 12 14
Gate Voltage (V)
H2 anneal only
RTP+H2 anneal
10-3
10-4
10-5
10-6
10-7
10-8
10-2
Tox
=11.1nm
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Direct Rapid Thermal Processing Plus D2 Annaeal: Improvement of Breakdown Voltage (30%) and Reduction of Leakage Current (100X)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 5 10 15
Gate Voltage (V)
D2 anneal only
RTP+D2 anneal
10-3
10-4
10-5
10-6
10-7
10-8
10-2
Tox
=10.2nm
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Direct Rapid Thermal Processing Plus D2 Anneal: Improvement of Breakdown Voltage (30%) and Reduction of Leakage Current (100X)
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
0 1 2 3 4 5 6
Gate Voltage (V)
D2 anneal only
RTP+D2 anneal
10-3
10-4
10-5
10-6
10-7
10-8
10-2 Tox
=3.7nm
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Direct Rapid Thermal Processing Plus D2 Anneal of Thin Oxides: Reduction of leakage current (105X)
This is similar to that of HfSiON (Gusev et al., IEDM Technical Digest, 451-454 (2001))
10-10
10-8
10-6
10-4
10-2
100
0 0.2 0.4 0.6 0.8 1 1.2
Control sample: D
2 anneal (30 min)
RTP (1 min) plusD
2 anneal (30 min)
105 X
Tox
=2.2nm
Gate Voltage (V)
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
Capacitance-Voltage Curves and oxide thickness of oxide measured before and after RTP
-2 -1 0 1 2
Control-1MHzRTP1min-1MHz-S1RTP1min-1MHz-S2RTP1min-1MHz-S3
350
300
200
150
100
50
0
Tox
=3.0 nm
250
Gate Voltage (V)
Table: Comparison of oxide thickness measured Table: Comparison of oxide thickness measured using using ellipsometryellipsometry before and after RTPbefore and after RTP
There is only a slight flatThere is only a slight flat--band voltage shift and thickness remains unchanged after RTP.band voltage shift and thickness remains unchanged after RTP.
Sample No.
Oxidation Parameters
Tox before RTP
Tox after RTP
#1110051
N2 @1000sccm, O2 @20 sccm, 9000C for 20 s
22.4 Å
23.3 Å
#0628052 N2 @1000sccm,
O2 @20 sccm, 9000C for 10 s
19.8 Å
20.9 Å
#1110052 N2 @2000sccm, O2 @20 sccm, 9000C for 20 s
19.5 Å
20.09Å
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
SummarySummary
•• We discovered a new effect, phonon energyWe discovered a new effect, phonon energy--coupling coupling enhancement, i.e. the energy coupling from the enhancement, i.e. the energy coupling from the SiSi--D bond to the D bond to the SiSi--SiSi TO mode and the TO mode and the SiSi--O rocking mode is dramatically O rocking mode is dramatically enhanced after the RTP processing directly on the oxide.enhanced after the RTP processing directly on the oxide.
• In addition to strengthening Si-D bonds, Si-O bonds are also strengthened. The breakdown voltage of oxides after RTP processing is improved by 30%.
• The leakage current of thin oxide (2.2 nm) after direct RTP processing is reduced by 105 times, similar to that of high-k oxides.
Dept. of Electrical and Computer Engineering University of KentuckyUniversity of Kentucky
2005 ISDRS, Bethesda, MD2005 ISDRS, Bethesda, MD
AcknowledgementsAcknowledgements
•• This research is supported by National Science Foundation This research is supported by National Science Foundation ECS-0093156 and EPS- 0447479.